INFRARED COMMUNICATION SYSTEMS

Abstract: A transmitter (10) and one or more receivers (12) are linked by infrared transmission via LED (14) and photodiode (16). In normal use, in-flight entertainment is modulated on left and right channels to give stereo sound on a user's earphones (28a, 28b). Transmitter switching means (18) can be operated to transmit a public address signal on one channel and a code signal on the other channel. At the receiver (12), the code is detected (at 36) to operate receiver switching means (38) which disable the user's volume controls (30), thus ensuring that the public address message is heard at full volume. Also described are improved infrared signal receiving arrangements mitigating the effects of ambient light interference.

Description: Infrared Communication Systems This invention relates to cordless communication systems using infra red light. The invention is particularly, but not exclusively, useful in public address and entertainment systems for aircraft, especially helicopters. It is customary in passenger-carrying helicopters to provide each passenger with a head set which acts as an ear defender. The head set receives background music and crew safety announcements. It is known to use an infra red cordless system to communicate with the individual head sets. The known systems have two disadvantages: 1 Each passenger has an individual volume control switch which may be turned low if the background music is unacceptable. However, a user with the volume turned low is likely not to hear safety announcements. 2 Communication can be lost because of interference by direct or reflected sunlight. In accordance with one aspect of the present invention, a communication system comprises a master station including a transmitter, and a plurality of receiving stations, each receiving station having a respective individual volume control; and the master station includes means for overriding the volume controls of the receiving stations. Preferably, the system uses infra red transmission between the transmitter and the receiving stations. Preferably, the system is a stereo system having two channels and the overriding means comprises: means in the master station for transmitting a code signal on one channel, and means in each receiving station responsive to said code signal to disable said one channel and to bypass the volume control of the other channel. The code signal is suitably a continuous monotone. From another aspect, the invention provides a receiving apparatus for use in an infra red communication system, the apparatus being in the form of a head set having left and right earpieces joined by a head band, the apparatus including a plurality of photodetectors positioned on the head set and facing in different directions. Preferably, there are four photodetectors positioned one at the front and one at the rear of each earpiece. Preferably, the apparatus includes circuit means operating to reduce the signal from a given photodetector as the light incident thereon increases. The circuit means suitably comprises a feed forward system. The feed forward system may suitably include, for each photodetector, a load element which provides a load resistance which varies inversely with the output current of the photodetector. Preferably, the load element is a bipolar tra: 3tor, and the system includes a constant curre__ϊ_ source connected in parallel with the photodetector to the transistor emitter. An embodiment of the invention will now be described, by way of example, with reference to the drawings, in which: Fig. 1 is a schematic block diagram illustrating one form of the first aspect of the invention; Fig. 2 is a plan view of a headset illustrating an embodiment of the second aspect of the invention; Fig. 3 is a circuit diagram illustrating a feed forward system used in the embodiment of Fig. 2; and Figs. 4a and 4b are graphs illustrating the operation of the circuit of Fig. 3. Referring to Fig. 1, a public address (PA) and in- flight entertainment (IFE) system for use in helicopters uses infrared (IR) transmission from a central unit 10 to a plurality of headsets such as 12. The central unit 10 includes one or more IR emitting diodes 14, and each headset includes one or more IR photodiodes 16. In normal use, left and right music signals are supplied from a source such as a tape deck via switching means 18a, 18b to left and right modulators 20a, 20b. The modulated signals are amplified by amplifier 22 and applied to the IR emitting diode 14 for transmission in the cabin space. The signal received at a given headset is detected by the photodiode 16 and receiver circuit 24. The left and right channel information is recovered by demodulators 26a, 26b and applied to right and left earpieces 28a, 28b via volume controls 30a, 30b and amplifiers 32a, 32b. The user may thus select a desired volume for listening to the IFE. When a PA announcement is to be made, the switching means 18a, 18b is operated to connect the right channel modulator 20a to a PA source (such as a microphone) and the left channel modulator 20b to a signal generator 34 producing a pure sine wave at a predetermined frequency. The demodulated sine wave at the output of the demodulator 26b in the headset is detected by a tone detector 36 which operates switching means 38a, 38b, 38c, 38d to switch the volume controls 30a, 30b out of circuit and to connect the demodulated PA signal from the demodulator 26a to both amplifiers 32a, 32b. Thus when a PA announcement is made, user control of listening volume is overridden and all headset users receive the announcement at a fixed volume. Turning to Fig. 2, the headset for use in the system comprises a headband 40 supporting ear shells 42a, 42b containing loudspeakers in the normal manner. The headset is provided with four receiving photodiodes 44, 46, 48, 50 arranged one forward and one aft on each of the ear shells 42a, 42b so as to provide four arcs of visibility of reception 44a, 46a, 48a, 50a. In the arrangement shown, the forward arcs and the aft arcs overlap along the centre line of the user's head. This is suitable for accommodating normal head movements in a cabin provided with IR transmitters on the forward and aft bulkheads. Other arrangements of visibility arcs may be used for other situations. In aircraft use, an IR system is subject to interference by strong sunlight. The interfering sunlight is likely to come from a single direction, typically through a cabin window, although interference from two directions is possible owing to reflection from a reflective surface within the cabin. However, interference from four directions is inherently unlikely. The headset of Fig. 2 is accordingly provided with circuit means for dealing with such interference, as will now be described. Fig. 3 illustrates an exemplary detector circuit used with one of the photodiodes 44, the other photodiodes each being provided with similar detector circuits. The photodiode 44 produces a current which is a function of the incident light energy and conventionally the signal for amplification would be recovered as the voltage across a load resistor on the photodiode anode. However, with such an arrangement strong sunlight interferes with reception in three modes, namely: (1) Strong light induces the current generator to produce large quantities of dc current which extend the voltage produced by the load beyond the supply voltage range; hence all information is lost. (2) The light will manifest broadband white noise generated at the most sensitive element in the circuit, i.e. before preamplification. (3) The large dc current being produced by the photodiode makes the photodiode less sensitive to light deviations, further interfering with the reception capabilities of the system. Of these, the first two can be mitigated by using dc active feed back and by reducing reception bandwidth, but the third mode cannot be reduced by these means. In Fig. 3, the photodiode 44 is connected in a feed forward system which acts to increase the transresistance of the system as the incident light increases. Specifically, the photodiode load includes the emitter-collector path of bipolar junction transistor Q2. As the photodiode current varies between typically 10 nA and 10 mA, the dynamic resistance of Q2 emitter will vary between approximately 250 kOhm and 25 Ohm. This result is desirable in itself, as it gives discrimination against high levels of noise induced by high light intensities. However, the value of 250 kOhm at low light levels would reduce the system bandwidth below 10 kHz and large circuit noises would also be induced. To meet this point, the emitter of Q2 is connected in parallel to the photodiode 44 and to a stabilized current source provided by transistor Ql. If the current supplied by Ql is set at, say, 6 μA then the largest value of the Q2 emitter resistance is set at approximately 4 kOhm, which is in accordance with the general requirements of the system. The circuit as thus far described gives a transresistance characteristic as illustrated in Fig. 4a. However, the upper limit of the photodiode current is around 10 mA, which would give a transresistance of about 2.5 Ohm. It would be beneficial to reduce the eventual transresistance to zero, and this is achieved using Q4 and R2. At some prescribed photodiode current, the voltage generated at R2 will exceed Vbe and thus reduce the output voltage of Q5 to zero. This results in a modified transresistance characteristic as shown in Fig. 4b, with the points 1 and 2 being readily set by choice of Rl and R2. The circuit of Fig. 3 has a high degree of immunity to fluctuations in dc supply voltage, very low circuit noise (typically about 800 nV) , a high bandwidth (3 dB limit typically 310 kHz), and low power consumption (typically 1.2 mW) . These advantages are in addition to the rejection of interference from ambient sunlight. Using surface mount technology, the circuit of Fig. 3 can be implemented with . low component count in an area of about 0.5 square inches.

Claims: CLAIMS 1. A communication system comprising a master station including a transmitter, and a plurality of receiving stations, each receiving station having a respective individual volume control; and the master station includes means for overriding the volume controls of the receiving stations. 2. A system according to claim 1, which uses infra red transmission between the transmitter and the receiving stations. 3. A system according to claim 1 or claim 2, which is a stereo system having two channels and the overriding means comprises: means in the master station for transmitting a code signal on one channel, and means in each receiving station responsive to said code signal to disable said one channel and to bypass the volume control of the other channel. 4. A system according to claim 3, in which the code signal is a continuous monotone. 5. A receiving apparatus for use in an infra red communication system, the apparatus being in the form of a head set having left and right earpieces joined by a head band, the apparatus including a plurality of photodetectors positioned on the head set and facing in different directions. 6. Apparatus according to claim 5, in which there are four photodetectors positioned one at the front and one at the rear of each earpiece. 7. Apparatus according to claim 5 or claim 6, including circuit means operating to reduce the signal from a given photodetector as the light incident thereon increases. 8. Apparatus according to claim 7, in which the circuit means comprises a feed-forward system. 9. Apparatus according to claim 8, in which the feed forward system, for each photodetector, includes a load element which provides a load resistance which varies inversely with the output current of the photodetector. 10. Apparatus according to claim 9, in which the load element is a bipolar transistor. 11. Apparatus according to claim 10, including a constant current source connected in parallel with the photodetector to the transistor emitter.

Inventors: SAMARAI LAITH FARIS, SAMARAI, LAITH, FARIS
Applicants: CALEDONIAN AIRBORNE SYSTEMS LI, SAMARAI LAITH FARIS, CALEDONIAN AIRBORNE SYSTEMS LIMITED, SAMARAI, LAITH, FARIS
Application year: 1994
Main classification: H04R27
IPC codes: H04B5, H04B10, H04R27